1. Field of the Invention
This invention relates to magnetic resonance imaging apparatus using NMR (nuclear magnetic resonance).
2. Description of the Related Art
Magnetic resonance imaging apparatus, by utilizing the resonance of atomic nuclei, visualize biological tissues based on spin relaxation time differences thereof. Such apparatus are extremely useful in morphological diagnosis in the medical field since they realize images of excellent contrast reflecting the relaxation time differences. Generally, NMR parameters used include proton density p and two types of relaxation time. The latter are longitudinal relaxation time (also called spin-lattice relaxation time) T1 and transverse relaxation time (also called spin-spin relaxation time) T2.
A contrast improving technique for the magnetic resonance imaging apparatus which relies on MT (magnetization transfer) effect has become known in recent years (see, for example, S. P. Wolf and R. S. Balaban, Mag. Reson. Med. vol. 10, p135, 1989; B. S. Hu et al., Mag. Reson. Med. vol. 26, p231, 1992; and Japanese Patent Publication (Unexamined) No. 3-173529). This technique acquires image contrast based on an interaction between protons of free water in biological tissue, and protons of macromolecules such as membranes and proteins and protons of water surrounding them in a movement-restricted manner (hereinafter called restricted water). The image contrast resulting from magnitude of MT is called MTC (magnetization transfer contrast). This MT effect is expected to benefit medical diagnosis not only with improved image contrast but by reflecting tissue properties.
Conventionally, this MTC image is acquired by a method which applies presaturation pulses having a frequency band slightly offset (i.e. having a frequency offset) from the resonance frequency of free water, or presaturation pulses called binominal pulses and having a special shape (these pulses being collectively called MTC pulses herein), to an imaging sequence of the gradient echo or spin echo technique. The binominal pulse is a pulse of special form having a zigzag-shaped frequency spectrum not including the resonance frequency of free water but including a frequency at which the protons of restricted water resonate (Hore, P. J., 1983, A new method for water suppression in the proton NMR spectra of aqueous solutions, J. Magn. Resonance, 54, 539-542).
The protons of restricted water, which rapidly relax transversely (e.g. in transverse relaxation time T2 not exceeding 0.5 msec) and which cannot be visualized by an ordinary method, can be caused to resonate partially and become saturated when irradiated with MTC pulses. The saturation of the protons of restricted water indirectly influences the strength of NMR signals acquired at this time from the protons of free water which slowly relax transversely (e.g. in transverse relaxation time T2 which is at least 30 msec), thereby to change image contrast.
However, when, as in conventional practice, RF pulses having a relatively high power are applied as MTC pulses, RF heating (proportional to a product of MTC pulse irradiation strength and irradiation time) is caused by MTC pulses including high frequency components. This poses a problem, particularly when the human body is examined.